1. Field of the Invention
The invention concerns a spaceborne scatterometer.
The invention concerns more specifically a wind-scatterometer.
2. Discussing of the Background
The primary mission of a spaceborne scatterometer is to determine wind speed and direction over the ocean. This is achieved by performing a set of radar cross-section measurements (referenced as .sigma..sup.0) at different azimuth view-angles over a resolution cell. Then, the relationship which relates the wind speed and direction to the radar cross-section, the so-called .sigma..sup.0 model, is inverted to extract the wind information. A minimum number of independent measurements per pixel (known as resolution cell) of three is required for an unambiguous wind extraction (three degrees of freedom).
This is realized by the current generation of scatterometers, such as:
ERS-1/2 AMI-Wind (which is described for example in the following publication: "The DataBook of ERS-1", ESA Publication ESA BR-75, April 1991); or PA1 NSCAT (NASA Scatterometer, on-board ADEOS-1 satellites series, description of which can be obtained, for example, from the following WEB page (February 1998): PA1 1/ Each of the antennas requires a free field-of-view, making their accommodation difficult on the spacecraft (antenna length is typically 3.5 to 4 m at C-band). PA1 2/ The deployment of the antennas is a critical operation. PA1 3/ A switch matrix is required in the radar front-end in order to commute between the different antennas. This introduces additional losses and a critical RF-path (single point failure). PA1 4/ The side-looking radar working principle does not allow the area under the satellite to be imaged. A large gap around the nadir is unavoidable as a consequence. This makes the global Earth coverage more difficult. PA1 1/ The high rotation speed of the antenna (1-meter-diameter reflector) poses mechanical reliability and spacecraft stability (high momentum) problems. PA1 2/ The concept would be difficult to apply at C-band as the antenna dimension would have to be multiplied by a factor of 2.5 (SeaWinds operates at Ku-band). PA1 3/ Only two azimuth view-angles per pixel are provided beyond 700 km (radius of the inner spot beam) on both sides of the sub-satellite track, which is a very serious drawback for wind retrieval (similar to SEASAT). PA1 4/ The speed of rotation is closely related to the spatial resolution and the number of spot beams. An improvement of the spatial resolution would require an increasingly complex instrument (larger antenna, higher rotation speed, more spot beams, i.e. more radar channels). PA1 a rapid global coverage of the Earth (within one to two days) which requires a wide measurement swath (typically equal or greater than 1500 km); PA1 an adequate spatial resolution to resolve ocean surface wind under various meteorological conditions (typically 25 to 50 km); and PA1 an accurate wind retrieval which requires a high radiometric accuracy and multiple azimuth view-angle (typically .gtoreq.3 independent) measurements.
http://www.eorc.nasda.go.jp/ADEOS/Project/Nscat.html; PA2 and a next generation of scatterometers, such as ASCAT, on-board METOP satellites series, as described in J. Kerkmann and D. Klaes, "Perspective for the Advanced Scatterometer (ASCAT) on METOP, " Proc. ESTEC Workshop on Emerging Scatterometer Applications, ESA Publication ESA SP-424, November 1998, pages 13-19. PA2 by use of three independent antennas (i.e. three independent beams) per imaged swath (for example 45.degree., 90 .degree. and 135.degree. azimuth orientations for AMI-Wind and ASCAT).
Another scatterometer, the SEASAT scatterometer (described for example in the article by D. Offiler, "A Comparison of SEASAT Scatterometer-Derived Winds with JASIN Surface Winds," Int. J. Remote Sensing, Vol. 5, 1984, PP. 365-378) had two beams per swath only, resulting in ambiguous wind direction which had to be removed using additional information from a weather model.
On the other hand, a next generation of scatterometers, such as SeaWinds (which is described for example in the article by M. W. Spencer et al.: "Tradeoffs in the Design of a Spaceborne Scanning Pencil Beam Scatterometer: Application to SeaWinds," IEEE Trans. Geosc. & Remote Sensing, Vol. 35, 1997, pp. 115-126) uses a conically scanning spot (pencil-)beam antenna to obtain two to four independent views per pixel depending on the swath position. The central part (1400 km) of the swath is characterized by four views whereas only two views are provided within the 200 km wide-edge-strips, resulting in a 1800 km total swath. This limitation of two views only within said edge-strips of the swath is due to the use of two spot beams for illuminating the ocean surface. The resulting ambiguity in wind direction must be dealt with by special ambiguity removal techniques.
Accordingly, there are two main categories of scatterometers: fixed, multiple beam scatterometers, and mechanically rotated spot (pencil-) beam antenna around a vertical axis, respectively.
Technical characteristics and drawbacks of said two categories of scatterometers will be now described in a more detailed manner.
A fixed, multiple beam scatterometer, such as ERS-1/2 AMI-Wind, NSCAT, ASCAT, and SEASAT scatterometers, uses deployable fan-beam antennas to image a swath parallel to the sub-satellite track. Once deployed, no mechanical control is required for the antennas, which simplifies the spacecraft attitude control. However, a number of disadvantages are associated with the concept:
For a non-ambiguous wind retrieval, a minimum of three azimuth look-angles per pixel are required. ERS-1/2 AMI-Wind uses three antennas for imaging a 500-km swath on the right-hand-side of the satellite. Both the NSCAT and ASCAT have a double imaged swath, requiring six antennas to be mounted. The SEASAT scatterometers also had a double swath, but were limited to two azimuth view-angles per imaged swath. Consequently, a directional ambiguity remained after applying the wind retrieval algorithm, which had to be removed using empirical methods.
The NASA SeaWinds scatterometer uses a mechanically rotated antenna around the vertical axis, which produces two spot (pencil-)beams sweeping in a circular motion (reflector antenna). The requirement to produce a continuous coverage imposes a minimum rotation speed of around 18-rpm (3.3 s rotation period). The problem of the swath gap around the nadir is effectively solved for this configuration at a price of the following disadvantages:
On the other hand, the desirable features of a wind scatterometer are: